Highly Efficient Arsenite [As(III)] Adsorption by an [MIL-100(Fe)] Metal–Organic Framework: Structural and Mechanistic Insights

The MIL-100­(Fe) metal–organic framework presents a high As­(III) uptake capacity of 120 mg g–1. Mechanistic insights into the role of Fe sites versus carbon sites on As­(III) uptake are provided by a comparative study of a series of MIL-100­(Fe) calcinated at 600, 800, and 900 °C. Using powder X-ray diffraction, TEM, scanning electron microscopy, and N2-porosimetry, we have mapped the morphology evolution of the materials. Fourier transform infrared spectroscopy, thermogravimetric analysis, and electron paramagnetic resonance show that noncalcined MIL-100­(Fe) bears Fe3+ atoms; however, after carbonization, a porous carbon matrix is formed bearing zero-valent iron cores coated with an Fe-oxide layer and iron carbide. The relative proportion of these phases depends on the calcination temperature, that is, 600, 800, and 900 °C. A comprehensive surface complexation model is presented, allowing a quantitative description of the As­(III) adsorption on Fe sites and carbon sites. More specifically, As­(III) uptake can be attributed to specific FeOH sites, located inside the pores and carbon C x OH2 sites located on the surface. Confinement inside the pores is found to be responsible for the lateral interactions among the adsorbed [H3AsO3] species. The As­(III) uptake of MIL-100­(Fe) is 3- to 10-fold higher versus pertinent adsorbent materials, such as graphite/graphite oxide, activated carbon, and pyrolytic carbon, and comparable with that of MIL-101­(Cr).